Obesity is a primary target for public health advocates keen on reducing an American "epidemic" in which more than one-third of U.S. adults and approximately 17 percent of kids and adolescents are obese.

Advocates often tout ways for people to reduce their risk for obesity, such as by eating healthily and setting weight loss goals, or they may support risk prevention for future generations, such as by recommending breast-feeding to new moms or exercise habits at young ages.

A new study, however, says some of these long-held recommendations are nothing more than myths.

"False and scientifically unsupported beliefs about obesity are pervasive," study author Dr. David Allison, associate dean for science in the School of Public Health at the University of Alabama at Birmingham, said in a written statement. "As health professionals, we should hold ourselves to high standards so that public health statements are based on rigorous science."

For the study, published online Jan. 30 in the New England Journal of Medicine, Allison and his team conducted Internet searches of popular media and scholarly scientific papers, to come up with a list of obesity-related myths that could have widespread public health implications if believed. The researchers identified seven myths they said were not scientifically-backed by evidence.

One myth researchers busted was that losing lots of weight rapidly may cause the weight to come back one day, whereas slow, gradual weight loss will accumulate over time to produce longer-lasting changes. The researchers found evidence that people who lose more weight rapidly are more likely to weigh less, even after several years.

The oft-told advice that people should set realistic weight loss goals or else they'll become frustrated and not lose weight -- advice that may be especially prevalent around New Year's -- is a myth, according to the study, which cited data that people may actually do better with more ambitious goals.

Other major myths busted include the idea that breast-feeding may reduce a child's risk for obesity. The researchers say this myth has been "passionately defended" for more than a century, but while breast-feeding may offer benefits to both mom and baby, evidence does not suggest preventing obesity is one of them.

That old adage that having sex is just like exercise, causing people to lose 300 calories? The researchers found people would be lucky to burn one-twentieth of that during sex, which offers about the same calorie expenditure from sitting on a couch, the researchers said. For example, a 154-pound man in his early-to-mid 30s who has sex for six minutes will expend about 21 calories during intercourse, they wrote.

Rounding out the busted myths were:

Myth: Small, sustained changes in how many calories we take in or burn will accumulate to produce large, long-term weight.

In fact, small changes in calorie intake or expenditure do not accumulate over time, the researchers said, and changes in body mass eventually cancel out changes in calorie intake or burning.

CBS News medical correspondent Dr. Jon LaPook explains that this myth is based off the idea that 3,500 calories equal one pound of fat, which means eating 100 fewer calories or burning 100 more calories each day over the course of a year (365 days) would amount to 10 pounds in lost weight.

"But many people don't realize that as calorie intake goes down and weight comes off, the body does the metabolic equivalent of turning down the furnace. It lowers the number of calories it burns each day in the course of normal living, and that makes it harder to keep losing" weight, LaPook told CBSNews.com.

That makes sense in terms of evolution, he added, because the body is prevented from using up fat stores when food is scarce.

"That may be good during famines but it's bad when food is plentiful and overweight people are trying to lose weight," LaPook said. "This may be a painful truth for my patients but I try to explain it so they will have more realistic expectations."

Myth: Patients who feel "ready" to lose weight are more likely to make the required lifestyle change, so health-care professionals need to measure each patient's diet readiness.

The researchers found readiness neither predicts weight loss nor helps to make it happen among those seeking weight-loss treatment after reviewing five studies.

Myth: Physical education classes play an important role in reducing and preventing childhood obesity.

Physical education, as typically provided, does not appear to counter obesity, according to the study. While physical activity could prevent or reduce obesity, the researchers said it may not be plausible to reach the duration and intensity required for weight loss at school.

In addition to busting myths, the researchers also found six "presumptions" that they say are widely believed but need more studies before drawing conclusions about weight gain.

Have you ever heard that skipping breakfast may lead to obesity? The researchers found two studies that found no obesity risk reduction from eating breakfast. Many advocates argue that eating a diet full of fruits and vegetables can bring about weight loss. But while extra veggies may bring health benefits, the researchers say simply stocking up on produce in the absence of other behavioral changes won't lead to weight loss.

Public health advocates often push for community-based weight loss interventions, such as building more parks or sidewalks, but studies that suggest a benefit have only been observational and don't show cause-and-effect, leading researchers to also call that a presumption.

There is also no rigorous evidence that learning exercise and eating habits as a child will influence weight gain throughout life, nor is there proof that snacking contributes to weight gain or obesity.

"Even observational studies have not shown a consistent association between snacking and obesity or increased BMI," the researchers write.

"From social media outlets like Facebook, to mainstream television news to dietetics and nutrition textbooks, these myths are perpetuated, irrespective of the scientific evidence," said study co-author Dr. Krista Casazza, an assistant professor in the department of nutrition sciences at the UAB School of Health Professions. "As scientists, we have the responsibility to present the evidence as it exists without inflating ideas and contributing to popular misconceptions. As a registered dietitian, I feel that providing evidence-based statements about weight loss is essential."

Then what about obesity do the researchers consider "fact?"

While genetics play a large role in obesity risk, the authors say it's a fact that heredity does not mean destiny, and environmental changes can lead to weight loss. Dieting, or reducing the number of calories you eat, is an effective way to lose weight but recommending that someone go on a diet generally does not work well for them in the long-term. This may sound obvious, the researchers wrote, but people who switch to eating healthier foods won't lose weight unless they take other actions to reduce their calorie intake.

Also, the researchers say it's a fact that exercise increases health, and enough of it could aid in long-term weight loss. Using meal-replacement products in a structured program also appears to be associated with more weight loss than holistic approaches based on moderation and variety, they added. Pharmaceutical weight loss agents and bariatric surgery can also lead to meaningful weight loss, they found.

Lastly, overweight children are more likely to shed pounds if the whole family is involved, the researchers found evidence to support.

"This is an especially compelling article for me as a practicing physician because it highlights the friction between 'belief' and 'evidence-based medicine,'" said Dr. LaPook. "It's hard to think of another area of health where there are more devoutly held beliefs with little or no objective support from well-designed studies."

The study was partly funded by the National Institutes of Health. Several of the researchers also disclosed financial relationships, some of which included payments or grant support from Coca-Cola, Kraft Foods, McDonald's Global Advisory Council, Vivus and Arena Pharmaceuticals -- makers of two obesity drugs that recently got FDA approval -- and Jenny Craig.

"It raises questions about what the purpose of this paper is" and whether it's aimed at promoting drugs, meal replacement products and bariatric surgery as solutions, Marion Nestle, professor of nutrition and food studies at New York University, told the Associated Press.

"The big issues in weight loss are how you change the food environment in order for people to make healthy choices," such as limiting soda sizes or junk food marketing to children, she said.